1. Field of the Invention
This invention generally relates to wireless communication antennas and, more particularly, to a system and method for tuning an antenna with the aid of a ferroelectric dielectric material.
2. Description of the Related Art
There are several types of conventional antenna designs that incorporate the use of a dielectric material. Generally speaking, a portion of the field that is generated by the antenna returns to the counterpoise (ground), from the radiator, through the dielectric. The antenna is tuned to be resonant at frequencies, and the wavelengths of the radiator and dielectrics have an optimal relationship at the resonant frequency. The most common dielectric is air, with a dielectric constant of 1. The dielectric constants of other materials are defined with respect to air.
Ferroelectric materials have a dielectric constant that changes in response to an applied voltage. Because of their variable dielectric constant, ferroelectric materials are good candidates for making tunable components. Under presently used measurement and characterization techniques, however, tunable ferroelectric components have gained the reputation of being consistently and substantially lossy, regardless of the processing, doping or other fabrication techniques used to improve their loss properties. They have therefore not been widely used. Ferroelectric tunable components operating in RF or microwave regions are perceived as being particularly lossy. This observation is supported by experience in Radar applications where, for example, high radio frequency (RF) or microwave loss is the conventional rule for bulk (thickness greater than about 1.0 mm) FE (ferroelectric) materials especially when maximum tuning is desired. In general, most FE materials are lossy unless steps are taken to improve (reduce) their loss. Such steps include, but are not limited to: (1) pre and post deposition annealing or both to compensate for O2 vacancies, (2) use of buffer layers to reduce surfaces stresses, (3) alloying or buffering with other materials and (4) selective doping.
As demand for limited range tuning of lower power components has increased in recent years, the interest in ferroelectric materials has turned to the use of thin film rather than bulk materials. The assumption of high ferroelectric loss, however, has carried over into thin film work as well. Conventional broadband measurement techniques have bolstered the assumption that tunable ferroelectric components, whether bulk or thin film, have substantial loss. In wireless communications, for example, a Q of greater than 80, and preferably greater than 180 and, more preferably, greater than 350, is necessary at frequencies of about 2 GHz. These same assumptions apply to the design of antennas.
Tunable ferroelectric components, especially those using thin films, can be employed in a wide variety of frequency agile circuits. Tunable components are desirable because they can provide smaller component size and height, lower insertion loss or better rejection for the same insertion loss, lower cost and the ability to tune over more than one frequency band. The ability of a tunable component that can cover multiple bands potentially reduces the number of necessary components, such as switches that would be necessary to select between discrete bands were multiple fixed frequency components used. These advantages are particularly important in wireless handset design, where the need for increased functionality and lower cost and size are seemingly contradictory requirements. With code division multiple access (CDMA) handsets, for example, performance of individual components is highly stressed.
It is known to use ferroelectric materials for the purpose of frequency tuning antennas. However, the use of FE dielectric materials has not always been effective, especially if the FE materials are not located in the regions of greatest electromagnetic filed densities. In the case of a conventional patch antenna, the region of greatest electromagnetic fields is between the radiator and the counterpoise (ground). As a result of ineffective FE dielectric placement, the changes in dielectric constant have a minimal effect on changes in the resonant frequency of the antenna. To achieve a useful change in resonant frequency, these conventional FE dielectric antennas have had to rely on multiple radiators.
It would be advantageous if the resonant frequency of an antenna could be selectable during use.
It would be advantageous if FE material could be used to control the resonant frequencies of an antenna.
It would be advantageous if the resonant frequency of an FE material antenna could be changed in response to applying a voltage to the FE material.
It would be advantageous if FE material antenna could be used to effectively change the resonant frequency of a conventional design antenna with a single radiator.
The present invention describes antennas fabricated with FE materials as a dielectric. The dielectric constant of the FE material can be controlled by an applied voltage. Because there is a fixed relationship between dielectric constant and resonant frequency, the resonant frequency of the antenna can be controlled using the applied voltage.
Accordingly, a method is provided for frequency tuning a single-band wireless communications antenna. The method comprises: forming a radiator; forming a dielectric with ferroelectric material proximate to the radiator; applying a voltage to the ferroelectric material; in response to applying the voltage, generating a dielectric constant; and, in response to the dielectric constant, communicating electromagnetic fields at a resonant frequency. Some aspects of the method further comprise: varying the applied voltage; and, modifying the resonant frequency in response to changes in the applied voltage.
Modifying the resonant frequency includes forming an antenna with a variable operating frequency responsive to the applied voltage. Alternately stated, forming an antenna with a variable operating frequency includes forming an antenna with a predetermined fixed characteristic impedance, independent of the resonant frequency.
In some aspects of the method forming a radiator includes forming a single-radiator.
In some aspects of the method forming a dielectric with ferroelectric material includes: forming the dielectric with a dielectric material from a first material having a fixed dielectric constant; and, forming the dielectric with the ferroelectric material having a variable dielectric constant. Then, modifying the resonant frequency includes modifying the resonant frequency in response to the varying the dielectric constant of the ferroelectric material.
In other aspects, forming a dielectric with ferroelectric material includes forming the dielectric with a plurality of dielectric materials, each from a material having a fixed dielectric constant. Alternately or in addition, forming a dielectric with ferroelectric material includes forming the dielectric with a plurality of ferroelectric materials, each having a variable dielectric constant.
Additional details of the above-described method and a family of antennas fabricated with a FE material dielectric are described below.